The present invention relates in general to circulatory assist devices, and, more specifically, to enhanced reliability and fault monitoring of motor components of an implanted pump unit.
Many types of circulatory assist devices are available for either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. The DuraHeart® system employs a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. An electric motor magnetically coupled to the impeller is driven at a speed appropriate to obtain the desired blood flow through the pump.
A typical cardiac assist system includes a pumping unit, electrical motor (e.g., a brushless DC motor integrated into the pump), drive electronics, microprocessor control unit, and an energy source such as rechargeable batteries and/or an AC power conditioning circuit. The system may be implantable, either fully or partially. The goal of the control unit is to autonomously control the pump performance to satisfy the physiologic needs of the patient while maintaining safe and reliable system operation. A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. Pat. No. 7,160,243, issued Jan. 9, 2007, which is incorporated herein by reference in its entirety. Thus, a target blood flow rate may be established based on the patient's heart rate so that the physiologic demand is met. The control unit may establish a speed setpoint for the pump motor to achieve the target flow. Whether the control unit controls the speed setpoint in order to achieve flow on demand or whether a pump speed is merely controlled to achieve a static flow or speed as determined separately by a physician, it is essential to automatically monitor pump performance to ensure that life support functions are maintained.
A typical pump motor employed for a blood pump is a three-phase permanent magnet electric motor that can be driven as a brushless DC or a synchronous AC motor without any position sensor. The need for a position sensor is avoided by controlling motor operation with one of a variety of methods that use the measured stator phase currents to infer the position. Vector control is one typical method used in variable frequency drives to control the torque and speed of a three-phase electric motor by controlling the current fed to the motor phases. This control can be implemented using a fixed or variable voltage drive delivered via an inverter comprised of pulse width modulated H-bridge power switches arranged in phase legs. Reliability, fault detection, and fault tolerance are important characteristics of an electrically-powered blood pump, drive system, and cable, and it would be desirable to improve each of them.
Co-pending application Ser. No. 13/418,447, filed Mar. 13, 2012, entitled “Fault Monitor For Fault Tolerant Implantable Pump,” which is hereby incorporated by reference, discloses a fault-tolerant inverter/cable system wherein redundant inverter legs are coupled to the motor phases by redundant, parallel conductors between the external unit and the implanted pump. For a three-phase motor, the redundant interconnect system includes six conductors in the cable. By monitoring the equality of the current and/or voltage of the two conductors on the same phase, a fault or impending fault can be detected for each individual conductor. However, this system redundancy does not continue into the motor phases and other components within the motor because of size and other limitations. In the event of a failure or degraded condition inside the pump motor (e.g., a soldering terminal failure, a coil wire breakage, damage to a flex circuit substrate, a coil turn-to-turn short, a layer-to-layer short, or a core/yoke detachment), the current/voltage on the conductors for a single phase remain about equal and no fault is detected. Therefore, further means of fault detection would be desirable.